MX records point to IP addresses

One of our users is having difficulty sending email to one of his clients. His client seems to have his MX record configured incorrectly because his MX record points to an IP address instead of a hostname.

Since exim by default rejects connecting to hosts with this type of MX record my user is unable to send emails to his client. Although I know that the remote hosts setup is not RFC compliant and that the fault for this problem lies on the remote host my user still looks to me for a resolution to this issue. He argues that the fault must be with us since he is able to send emails to his client using his yahoo mail.

I did a google search and I have found that adding allow_mx_to_ip = true to my exim.conf file will fix this problem.

Regardless of where the fault of the issues lies, I do want to keep my users happy but I am a little hesitant on adding the allow_mx_to_ip to my exim.conf since this is essentially a bypass of being RFC compliant and I strongly feel that the responsibility to fix this issue is with the remote host.

Now my question

I just wanted to get everyones opinion on how they handle a situation like this? And if anyone has added allow_mx_to_ip to their conf, are there any drawbacks or possible security issues to consider? So am I being a little pig headed or is allowing this configuration a standard thing?

Personally, I would not enable this. MX records are suppose to point to hostnames, not IP addresses. In my opinion this is a problem with the other server and the administrators of that server need to be aware of this and make the appropriate adjustment.

If you cave into everyone's wanting and demands, then you aren't really teaching anything. The administrators of this server may continue to use IP addresses for their MX records, which is just wrong. The administrators need to know that this is not acceptable and that they are the ones that will need to change. If this is not enforced, then really what is the point of having any standards? You'll be making exceptions for every server. Standards are written so that exceptions do not have to be made or in any case, very limited. This may come across as mean, but my stance on this and issues like this, is that a line has to be drawn somewhere. Standards are in place that say an MX record should point to a hostname not an IP, unless someone can give a valid reason for why an MX record should point to an IP, or why this specific MX record cannot point to a hostname, then I would say to draw the line at the governing standards.

That being said, how many MX records are available for this domain in question? Are all of the MX records pointing to an IP? Is the domain listed at RFC Ignorant?

The remote host has two MX records. The first (priority 0) is set to an IP address as discussed but the other is set to a hostname. Funny thing is, the second MX record gives me a 550 No Such User error when I try to sent email. The bad IP MX record however works fine if I send an email manually using their IP through telnet. It seems that they have all sorts of problems.

I checked RFC Ignorant and they are not listed there but I took a few minutes and added them to the list.

We are still up in the air on how we are going to handle our policy regarding this issue. I am surprised that large email services such as yahoo give in to such behavior. This makes it hard for little guys like us to follow standards when large organizations do not.